Chlamydia trachomatis is the cause for the most common bacterial sexually transmitted infection in the United States. Commonly referred to as a silent epidemic, C. trachomatis infections in women are frequently asymptomatic, and often go unnoticed and untreated. The infection can persist for months or years and ultimately cause pelvic inflammatory disease, ectopic pregnancies, and infertility. In order to establish persistent infections, C. trachomatis must subvert both the innate and the adaptive branches of the immune system. A central node that connects adaptive and innate immunity to C. trachomatis is the cytokine gamma-interferon (IFN?). This cytokine is produced by lymphocytes such as T cells of the adaptive immune system and induces cell-intrinsic, innate host defenses against Chlamydia in epithelial cells. In order to overcome IFN?-mediated immunity and replicate inside human epithelial cells, C. trachomatis evolved counterdefenses specifically adapted to its human host. These C. trachomatis counterdefenses and the corresponding repertoire of IFN?-inducible anti-Chlamydia defense programs are poorly characterized. Our goal is to decipher this interplay between IFN?-mediated host defense and chlamydial counterdefense, as such knowledge holds the potential to instruct the design of improved treatment options. To achieve this goal under Aim1, we designed unbiased functional genomics screens to identify human IFN?-inducible genes (ISGs) that exert potent anti-Chlamydia defenses. In parallel we employ defined C. trachomatis mutant libraries in functional bacterial genetic screens to identify C. trachomatis genes that subvert ISG- driven defense pathways. As proof-of-principle we already identified i) human ISGs with not-previously- described anti-Chlamydia activities and ii) C. trachomatis mutants that are hyper-susceptible to IFN?- activated host defenses in human cells. These novel human defense pathways and the corresponding C. trachomatis evasion mechanisms will be characterized through multidisciplinary approaches in Aim1. As an interrelated second aim, we will dissect the role of IFN? in C. trachomatis-induced inflammation, which underlies Chlamydia-associated diseases. Although best known as a potent inducer of cell-intrinsic immunity, IFN? also functions as a critical regulator of inflammation and associated diseases. We discovered that specific members of the superfamily of IFN?-inducible dynamin-like GTPases control Chlamydia-induced activation of inflammasomes, a class of cytosolic immune sensors that drive inflammation. Using organoid and other mouse and human cell culture systems as well as novel in vivo mouse models, we will in Aim2 define the function of IFN?-inducible GTPases in Chlamydia-triggered inflammation and in vivo pathogenesis. These studies will reveal critical insights into the role of IFN? in the immunopathology of Chlamydia infections with the potential to guide the development of new therapeutic or prophylactic treatments or vaccine designs.